1. Field of the Invention
The Stirling cycle has received attention since its invention early in the nineteenth century. For various reasons it has not achieved commercial success; but because of its high theoretical efficiency and inherently low pollution, it is currently the subject of a considerable research and development program directed primarily towards automotive use.
In modern Stirling technology a number of factors have arisen which have lead to continuing and formidable problems, a number of which have proved to be intractable in the practical sense.
Some of these problems are:
1. Power output is changed by changing the pressure of the working fluid; and this leads to a complex system for withdrawing working substance from the engine, and putting it back in almost instantaneously.
2. Almost all the heat loss is by direct cooling of the so-called "cold space" of the engine, which leads to difficult design problems.
3. The use of a light gaseous working substance raises problems of explosion (hydrogen), loss through high volatility, and in the case of helium, availability and cost.
4. The most difficult problem has to do with heat transfer, that is, getting the heat into the working substance, and to attaining satisfactory efficiency.
A type of Stirling engine presently the focus of much attention is the valveless type. It is not a true Stirling engine. It is better described as a pseudo-Stirling engine; and in it the admirable principle of the true Stirling engine (constant temperature expansion and constant temperature compression at a much lower temperature) has been sacrificed for mechanical simplicity. The engine has no valves controlling the expansion and compression in the cylinder. Each cylinder head is connected with the base of the next cylinder through a heat exchanger so that the pressure in the "hot space" of one cylinder and the "cold space" of the next cylinder to which it is connected must always be the same but is changing constantly.
The ideal efficiency of the true Stirling engine is (T.sub.2 - T.sub.1)/T.sub.2 where T.sub.2 is temperature of expanding gas or vapor; T.sub.1 is temperature during compression (in absolute degrees). This is the maximum or "Carnot" efficiency of a heat engine working between the temperature limits T.sub.2 and T.sub.1.
A true Stirling engine receives heat only during expansion. In a reciprocating engine this is usually done by heating the cylinder.
In the valveless type of Stirling engine, some heat is put into the working substance while it is expanding, but most is put in before expansion when the working substance is passing from the heat exchanger to the "hot space" above the piston. Thus, the engine actually carries out expansion somewhere between isothermal and isentropic; similarly for compression in the "cold space" below the piston.
A tube bundle can be used in both hot and cold spaces to increase heat transfer surface; but this cannot affect heat transfer during expansion or compression -- only ensure that the gas is at or close to maximum temperature before expansion, and at minimum temperature before compression.
Thus, the "pseudo" Stirling engine has, by its nature, to depart considerably from theoretical Stirling efficiency.
To minimize this effect, the valveless Stirling engine employs very high temperature on the "hot side" of the engine. Heat input to the working substance is effected by gaseous combustion products applied directly to the expansion cylinders. However, heat transfer at the hot gas-metal interface is always relatively poor. The combination of high temperature plus the oxidizing medium provided by the combustion gases require the use of exotic and expensive heat resistant alloys with large nickel-chromium content.